1. Field of the Invention
The present invention relates to equipment and a method for producing spark plugs. More particularly, the present invention relates to an equipment and a method for producing a spark plug including an insulator having an axial through-hole in which a metallic terminal fixed at an end portion thereof and a center electrode fixed at the other end thereof and a sintered conductive material member such as a conductive glass seal layer or a resistor being formed within the through-hole between the metallic terminal, and the center electrode to establish their electrical connection.
2. Field of the Invention
Conventional spark plugs for use in internal combustion engines have built-in resistors with a view to suppressing the generation of noise from electrical waves. Such spark plugs with a built-in resistor includes an insulator having an axial through-hole with a metallic terminal fixed at an end thereof and a center electrode fixed at the other end thereof and the resistor provided in the through-hole between the metallic terminal and the center electrode. A conductive glass seal layer is usually provided between the resistor and the metallic terminal or between the resistor and the center electrode to connect them electrically.
A typical method for producing the spark plug with a built-in resistor is shown in FIGS. 23A and 23B. In short, a center electrode 3 is inserted into a through-hole 6 in an insulator 2 and, thereafter, a conductive glass powder, a bulk powder of resistor composition and another mass of conductive glass powder are packed in that order. Finally, a metallic terminal 13 is pressed into the through-hole 6 from the side opposite to the center electrode 3, thereby making a spark plug assembly. Thus, the through-hole 6 in the insulator 2 contains a conductive glass powder layer 26 therein, a resistor composition powder layer 25 and another conductive glass powder layer 27 that are superposed one on another in that order from the center electrode 3. The spark plug assembly having this layer arrangement is brought into a heating furnace where it is heated to a temperature higher than the glass softening point. Thereafter, the metallic terminal 13 is uniaxially pushed in from the side opposite to the center electrode 3, whereby the respective layers 25 to 27 are compressed to form glass seal layers 16 and 17 and a resistor 15 as shown in FIG. 23B.
In the above-described method for the production of spark plugs with a built-in resistor, the individual layers are compressed by a so-called "one-side press" method. Namely, the metallic terminal 13 is pushed in toward the center electrode 3 with the position of the latter fixed. In this case, the conductive glass powder layer 26 located in the lowest position in FIG. 23A does not receive a sufficient pressing force due to the friction between the overlaying packings and the inner surface of the through-hole 6. Accordingly, the compression or flow of the powder is inhibited, sometimes causing poor sintering of the glass seal layer 16 on account of its low density. If this situation occurs, the carbon in the glass seal layer 16 is burnt away or the metallic component is oxidized. As a result, the electrical continuity between the resistor 15 and the center electrode 3 via the glass seal layer 16 becomes imperfect and as the use of the spark plug is prolonged. It may be that the conduction resistance increases to potentially cause occasional misfiring.